1. Technical Field
This invention relates to weather radar systems and, more particularly, to weather radar systems with path attenuation detection.
1. Description of the Related Art
Referring to FIG. 1A, a prior art weather radar system 10 is shown having a transmitter 12, a receiver 14, a display 16 with radials 18 and a common antenna 20 for transmitting pulses 17 to, and receiving echo pulses 19 from, the target, such as a precipitation target (not shown). A controller 22 interfaces with each of the transmitter 12, receiver 14 and display 18 as well as with an operator through manual inputs (not shown). The controller 22 includes an accumulator (not shown) which is sequentially incremented in range to generate a path attenuation compensation, or PAC, value with which to correct the displayed information. If the amount of attenuation compensation required exceeds a preset value based upon system capability for the selected parameters, a PAC alert discrete memory element is set and the display 18 alerted through the controller 22 to represent a PAC alert warning on the range radial 18 of interest.
The controller 22 provides the prior art radar system 10 with programmable parameters including transmitter pulse width, receiver bandwidth, initial STC (sensitivity time control) value and PAC (path attenuation compensation) threshold. In addition to input from the receiver 14 and transmitter 12, the controller 22 includes a PAC alert circuit 24. The controller 22 is able to change these programmable parameters on essentially a PRF (pulse repetition frequency) to PRF basis.
In order to maximize range resolution of the radar return from a target, such as precipitation, it is necessary to transmit the minimum pulse width. On the shorter selected ranges of a radar, the pulse width is optimumized for the required range coverage using the minimum transmit pulse width to preserve the best range resolution. If RF path attenuation exists due to intervening precipitation, then sufficient return may not be available from the farther range targets to detect and display them. When this occurs, it is standard practice to alert the operator to this possibility of reduced performance by marking the radial 18 of occurrence on the display 16 using information from far range bins (not shown) of the controller 22. The far range bins are random access memories which store data at the maximum displayed range. When the data from the far range bins are used for display, this is detected, to provide the display with a contrasting color. While alerting the operator to such a situation with this contrasting color is somewhat helpful, it still does not solve the problem of the loss of displayed information. In fact, the operator has no idea of what meteorological conditions might exist behind the displayed target, if the path attenuation is severe.
Unfortunately, even if the path attenuation becomes excessive as determined by the PAC alert circuit 24 within controller 22 as a result of the precipitation target returns, the system is unable to automatically improve performance. Instead operator intervention is required to manually increase sensitivity. Such manual adjustment often results in loss of far range calibration within the constraints of the maximum performance limitations of the system under condition of severe path attenuation.
Various attempts have been made to automatically make adjustments to contain other parameters to improve performance. In U.S. Pat. No. 4,370,652 of Lucchi issued Jan. 25, 1983 for "Control Systems for Radar Receivers", a radar control system is shown in which the receiver bandwidth and the gain of the receiver is automatically controlled by means of a computer, or discrete controller. The receiver gain is adjusted on the basis of the depth of the target received in order to improve long range detection capability. If long range targets have sufficient depth there is a detection advantage. The receiver bandwidth is adjusted on the basis of the received return time width, and the receiver gain is adjusted on the basis of the return time of the signal. The longer the return time, the narrower the receiver bandwidth can be made, and the greater the sensitivity, or gain.
A radar system with incremental AGC is shown in U.S. Pat. No. 4,680,588 of Cantwell issued July 14, 1987 for "Radar Systems With Incremental Automatic Gain Control". The AGC setting is adjusted by 6 db for each range cell in a next sweep based on the AGC setting of each corresponding range cell measurement in the present sweep. The gain of the receiver is changed on a bin to bin basis in order to maintain the receiver dynamic range within that presented by the signal. This is necessary in order to prevent receiver saturation which would distort the target information while maintaining a maximum sensitivity to lower level, target returns. Disadvantageously, this approach requires excessive hardware and software requirements and each incremental change must be done in real time.
In U.S. Pat. No. 3,149,332 of Kocher issued Sept. 15, 1964 for "Radar Isoecho Contour Sensitivity Gate", gain changes are switched into the receiver in order to change the receiver threshold and provide isoecho contour, i.e. a calibrated output, which improves the information displayed to the operator.
In each of the U.S. Pat. Nos. 3,525,095 of Cordry issued Aug. 18, 1970; 4,435,707 of Clark issued Mar. 6, 1984 and 4,529,983 of Lyall issued July 16, 1985, means for path attenuation compensation are provided. Unfortunately, compensation appears to be provided only within the receiver gain control range, and none appear to address the problem occurring when the system reaches maximum gain, or sensitivity, prior to display of the maximum range and can no longer compensate for path attenuation.
Disadvantageously, none of the above prior art radar systems adjust the transmitted pulse width or the receiver bandwidth as a function of path attenuation of the weather target in order to preserve maximum range resolution while having the advantage of maximum system detection capability, if a severe path attenuating circumstance is encountered.